High temperature superconductor (HTS) materials provide a means for carrying extremely large amounts of current with extremely low loss. HTS materials lose all resistance to the flow of direct electrical current and nearly all resistance to the flow of alternating current when cooled below a critical temperature. The development of HTS wires (the expression “wires” is used here for a variety of conductors, including tape-like conductors) using these materials promises a new generation of high efficiency, compact, and environmentally friendly electrical equipment, which has the potential to revolutionize electric power grids, transportation, materials processing, and other industries. However, a commercially viable product has stringent engineering requirements, which has complicated the implementation of the technology in commercial applications.
In the second generation HTS wire (coated conductor) technology, currently under development, the HTS material is generally a polycrystalline rare-earth/alkaline-earth/copper oxide, e.g. yttrium-barium-copper oxide (YBCO). The current carrying capability of the HTS material is strongly related to its crystalline alignment or texture. Grain boundaries formed by the misalignment of neighboring crystalline superconductor grains are known to form an obstacle to superconducting current flow, but this obstacle decreases with the increasing degree of alignment or texture. Therefore to make the material into a commercially viable product, e.g. an HTS wire, the superconducting material must maintain a high degree of crystalline alignment or texture over relatively long distances. Otherwise, the superconducting current carrying capacity (critical current density) will be limited.
A schematic of a typical second-generation HTS wire 100 is shown in FIG. 1. The wire includes substrate 110, buffer layer 120 (which could include multiple buffer layers), superconductor layer 130, and cap layer 140, and is fabricated as described below. It should be noted that in this and all subsequent figures, the dimensions are not to scale. Superconductor materials can be fabricated with a high degree of crystallographic alignment or texture over large areas by growing a thin layer 130 of the material epitaxially on top of a flexible tape-shaped substrate 110 and buffer layer 120, which are fabricated so that the surface of the topmost layer has a high degree of crystallographic texture at its surface. When the crystalline superconductor material is grown epitaxially on this surface, its crystal alignment grows to match the texture of the substrate. In other words, the substrate texture provides a template for the epitaxial growth of the crystalline superconductor material. Further, the substrate provides structural integrity to the superconductor layer.
Substrate 110 and/or buffer 120 can be textured to provide a template that yields an epitaxial superconductor layer 130 with excellent superconducting properties such as high critical current density. Materials such as nickel, copper, silver, iron, silver alloys, nickel alloys, iron alloys, stainless steel alloys, and copper alloys can be used, among others, in the substrate. Substrate 110 can be textured using a deformation process, such as one involving rolling and recrystallization annealing the substrate. An example of such a process is the rolling-assisted biaxially textured substrate (RABiTS) process. In this case large quantities of metal can be processed economically by deformation processing and annealing and can achieve a high degree of texture.
One or more buffer layers 120 can be deposited or grown on the surface of substrate 110 with suitable crystallographic template on which to grow the superconductor layer 130. Buffer layers 120 also can provide the additional benefit of preventing diffusion over time of atoms from the substrate 110 into the crystalline lattice of the superconductor material 130 or of oxygen into the substrate material. This diffusion, or “poisoning,” can disrupt the crystalline alignment and thereby degrade the electrical properties of the superconductor material. Buffer layers 120 also can provide enhanced adhesion between the substrate 110 and the superconductor layer 130. Moreover, the buffer layer(s) 120 can have a coefficient of thermal expansion that is well matched to that of the superconductor material. For implementation of the technology in commercial applications, where the wire may be subjected to stress, this feature is desirable because it can help prevent delamination of the superconductor layer from the substrate.
Alternatively, a non-textured substrate 110 such as Hastelloy can be used, and textured buffer layer 120 deposited by means such as the ion-beam-assisted deposition (IBAD) or inclined substrate deposition (ISD). Additional buffer layers 120 may be optionally deposited epitaxially on the IBAD or ISD layer to provide the final template for epitaxial deposition of an HTS layer 130.
By using a suitable combination of a substrate 110 and one or more buffer layers 120 as a template, superconductor layer 130 can be grown epitaxially with excellent crystal alignment or texture, also having good adhesion to the template surface, and with a sufficient barrier to poisoning by atoms from the substrate. The superconductor layer 130 can be deposited by any of a variety of methods, including the metal-organic deposition (MOD) process, metal-organic chemical vapor deposition (MOCVD), pulsed laser deposition (PLD), thermal or e-beam evaporation, or other appropriate methods.
A cap layer 140 can then be added to the multilayer assembly, which helps prevent contamination of the superconductor layer from above. The cap layer 140 can be, e.g., silver or a silver-gold alloy, and can be deposited onto the superconductor layer 130 by, e.g., sputtering. The cap renders the superconductor layer substantially impervious to contamination by environmental factors, which can degrade its electrical performance. The cap layer may also substantially prevent infiltration of the assemblies by surrounding cryogenic fluid, which can form balloons that can potentially mechanically damage the superconductor layer. Additionally, the cap layer also preferably “wets,” or promotes adhesion to, filler material used in a later step to optionally bond the assembly to stabilizer strips.
A subsequent oxygenation step converts the as-deposited superconductor material to the superconducting phase. Metal “stabilizer” strips, such as copper or stainless steel layers, can subsequently be bonded to the cap layer and to the substrate, e.g., by soldering. Stabilizer strips are discussed below, and in the incorporated patent references.
An exemplary as-fabricated multilayer HTS wire 100 includes a biaxially textured substrate 110 of nickel with 5% tungsten alloy; sequentially deposited epitaxial buffer layers 120 of Y2O3, YSZ, and CeO2; epitaxial layer 130 of YBCO; and a cap layer 140 of Ag. Exemplary thicknesses of these layers are: a substrate of about 25-75 microns, buffer layers of about 75 nm each, a YBCO layer of about 1 micron, and a cap layer of about 1-3 microns. HTS wires 100 as long as 100 m, and with widths of 10 cm or more, have been manufactured thus far using techniques such as those described above.
Although the formation of a cap layer by a physical vapor deposition process such as sputtering or evaporation is a well-understood process, its use can substantially increase the cost of producing HTS wires. Among other things, it can be relatively time intensive to sputter a sufficient thickness of metal to form a cap layer, e.g., 1-3 microns of silver. Thus, sputtering can slow wire production, and thus increase costs. It can also be relatively expensive to maintain a sufficient vacuum during the time required for sputtering. Moreover, sputtering is a relatively inefficient process. For example, typically approximately 30% of the source material consumed by the sputtering process may be deposited onto the superconductor layer and thus form the cap layer; while the remaining approximately 70% of the source material may be deposited on the inside of the vacuum chamber, or pulled into the vacuum itself. This inefficiency adds to the cost of performing the step, and, for more costly metals such as gold, may make the use of the process prohibitively expensive.
Heating during the physical vapor deposition of a cap layer can also have detrimental effects on the self-field and in-field performance of the superconductor, e.g., YBCO, thus necessitating active cooling of the wire during this step. This can add to the time and cost associated with producing the HTS wire, and presents a risk of degradation in the wire's performance.